In the art of dispensing cartridges it is common practice at present, for example, to provide, in particular, the dental profession with such devices which are pre-loaded with cements, filling material and the like. The cartridges are mounted in ejector-type holders having operating handles or levers and, by operation thereof, desired amounts of the material in the cartridges are discharged readily and precisely.
One example of a cartridge of interest to the present invention comprises the subject matter of prior U.S. Pat. No. 4,391,590 to Dougherty, dated July 5, 1983, and assigned to the assignee of the present invention. In the patented structure, the body of the cartridge is cylindrical and of uniform inner diameter and in which a discharge piston is mounted in position in the open filling end of the cartridge to form both a closure and an ejecting piston. Cartridges of this type are miniature and, by way of example, have an outer diameter of about one quarter inch and an inner diameter of approximately one eighth inch. The piston, accordingly, has a complementary outer diameter of about one eighth inch and at present is composed of solid plastic material.
It has been found in operating a device of this type that, when the piston is of a solid nature, shrinkage occurs in the molded product and is of such nature that when operated in the tubular body, material to be discharged, when under pressure in the cartridge by operation of the ejector holder, can escape between the inner wall of the cartridge and outer wall of the piston to an undesirable extent.
In an effort to obviate this difficulty, it now has been found that when the piston of desired outer diameter relative to the inner diameter of the body is formed with a somewhat hollow section, such as a somewhat thin cylindrical skirt or sleeve extending outward from the pressure end of the piston, the shrinkage in the outer diameter is minimal and negligible.
To adapt this arrangement to the piston for the tubular body of the cartridge, it has been found necessary to include details which appear to be novel and full description of which details are set forth below and also are illustrated in the accompanying drawing.